Numerous attempts have been made to produce plastic separators both reinforced and unreinforced, and many such attempts have been successful to the extent that the separators are commercially available and used within the industry concerned. These separators include blanks of cellulose fibers; for example, paper impreganated with resins insoluble in sulfuric acid or they comprise woven fabrics of polyvinylidene chloride and other plastic fibers. They also include glass fibers alone or in combination with other materials such as diatomaceous earth. These separators have many disadvantages and imperfections, which include porosity that is too low leading to high internal resistance in the battery which is undesirable, and pore size that is sufficiently large to permit shorting between the negative and positive plates or material transfer, such as antimony, which reduces the capacity of the battery.
Other separators in existence consist of unreinforced plastic material. These have preferably been prepared by mixing powdered thermoplastic material with a porous or pore producing material and a solvent to make the plastic processable. After a dough has been made, this is formed by extrusion or calendering into a sheet of suitable thickness and the solvent evaporated. For instance, see Witt et al U.S. Pat. No. 2,772,322. The Witt et al process preferentially uses an evaporative removal of the solvent. This evaporative removal is difficult to accomplish without the aid of elaborate mechanisms to support the dough which, at this point, has little strength. Furthermore, evaporation of the solvent from the surface of the dough causes undesirable distortion of the sheet of dough. If the pore forming material is porous, as in the case of diatomite, the sheet after evaporation need only itself be cut to the desired size and used as a separator. In this case, however, stresses are set up in the plastic structure which cause shrinkage and reduce the effective porosity of the product.
If, on the other hand, the pore forming material consists of a water soluble material such as an inorganic salt or organic material such as starch or sugar, this must be leached from the extruded or calendered sheet and the result is a porous sheet which is then dried. U.S. Pat. No. 3,055,966 describes one such process in which the pore making material consists of an inorganic salt and starch. However, separators, the porosity of which is caused by pore formers that have been leached or washed out, tend to have relatively low residual porosity due to shrinkage during such processing. Also, these separators have poor wetting characteristics which tend to retard passage of electrolyte and thereby cause high internal resistance when assembled in a battery.
A principal object of the present invention is to provide a process for producing a microporous member useful, for example, as a separator for electric storage batteries, with high porosity and good and permanent wetting characteristics, which separator is sufficiently rigid and impact resistant to be inserted by machinery between the battery plates.
Another object is to provide an improved process for producing a microporous member which minimizes the undesirable distortion of the sheet caused by evaporation of the solvent from the sheet.
Further, the separator should have sufficient dimensional stability, mechanical strength, and insolubility so as to maintain its dimensions and characteristics during service in an electric storage battery.
These and other objects of the invention, as well as the nature, operation and use of the invention, will become more fully apparent from the subsequent description and appended claims. All amounts and proportions of materials are expressed herein on a weight basis unless indicated otherwise.